The present invention relates to a method for installing a radial geothermal energy probe field.
The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention.
The use of geothermal energy for energy generation has increased significantly over the past years. Geothermal energy is typically generated either by using thermal energy probes or geothermal energy collectors installed in the ground. In the presently most widely used method today for using geothermal energy, a number of vertical bore holes arranged with a defined spacing from one another, in which individual geothermal energy probes are inserted, are installed in a defined area, for example a garden of a single family dwelling. Disadvantageously, with this type of installation, geothermal energy probes can only be installed in areas which are not built-up, because a corresponding drilling rig must be positioned at all locations where a geothermal energy probe is to be inserted. Introduction of a plurality of vertical geothermal energy probes is also relatively expensive, because the drilling rig used to drill the bore hole in the ground and insert the probe into the bore hole must be aligned anew at each of these locations. Transporting the drilling rig from the individual locations may also cause significant damage to the vegetation which has to be repaired later.
These disadvantages have lead to the development of a method where the geothermal energy probes are introduced into the ground radially, i.e., in different directions and with different inclination angles, from a single point, for example an excavated starting shaft. This type of star-shaped installation of geothermal energy probes is generally referred to as “Geothermal Radial Drilling” (GRD). This method has the significant advantage that the drilling rig must only be positioned at a single location from which the rig then introduces the bore holes in the ground with different directions. Due to the inclined arrangement of the thermal energy probes in the ground, these probes can moreover extend into regions of the ground where the surface has been built up.
Geothermal energy probes may not only be used to generate heat, but also to cool the air in buildings during the summer through heat exchange from the geothermal energy probes in the ground, when the temperature of the air and inside the buildings connected to the geothermal energy system is significantly higher than the temperatures in the ground.
One problem that has to be taken into account when designing geothermal energy fields where several geothermal energy probes are installed with a relatively small spacing therebetween is the reduced energy removal efficiency caused by the drift of the cooled (or heated) groundwater due to groundwater flow. Regardless if the groundwater flow in the corresponding region has a stable direction or an instable direction, the individual geothermal energy probes produce (heat or) cold streaks which can flow to other geothermal energy probes in the probe field and thereby significantly weaken the thermal efficiency of these geothermal energy probes exposed to the flow.
To reduce the mutual interaction between the geothermal energy probes, a greater spacing between the individual geothermal energy probes and/or a greater length of the geothermal energy probes are typically selected in a vertical installation than would otherwise be required based on a computation which takes into account the dimensioning parameters (in particular the energy demand and temperature gradients in the ground). Moreover, if the groundwater flow is directionally stable, the location of the individual geothermal energy probes can be selected such that groundwater whose temperature was changed by one of the geothermal energy probes does not flow towards another geothermal energy probe, or only to very few geothermal energy probes in the geothermal energy field. Such arrangement of the locations, however, can in many situations not be reliably determined because, on the one hand, the groundwater flow frequently lacks the required directional stability and, on the other hand, the drift of the groundwater cannot be predicted with sufficient accuracy.
The corresponding measures for reducing the mutual interaction between the geothermal energy probes of a radial geothermal energy probe field may involve increasing the separation angle between the individual geothermal energy probes and/or lengthening the probes.
A design of a geothermal energy system which matches as exactly as possible the demand is of vital importance, because an undersized system makes it impossible to satisfy the energy demand, whereas an oversized system results in unnecessarily high costs. However, a correct design of a geothermal energy probe field is quite complicated because a large number of factors affect the heat removal capacity of the installed geothermal energy probes. In general, in the design of a geothermal energy system initially an energy demand is defined which depends on the intended use (e.g., heating a single-family dwelling). This energy demand is converted into a corresponding total heat removal capacity of the geothermal energy system. The geothermal energy probe field can then be designed based on the determined total heat removal capacity, wherein the important factors are the number of geothermal energy probes, the length of the individual geothermal energy probes and—due to the mutual thermal interaction—the distance between the geothermal energy probes (in vertical geothermal energy probe fields) or the installation directions of the individual geothermal energy probes (in radial geothermal energy probe fields). By changing these factors, the thermal heat removal capacity of the geothermal energy probe field can be altered and adapted, wherein additional factors which can only be slightly affected or not at all, also considerably affect the specific heat removal capacity and must therefore be taken into consideration when designing the geothermal energy probe field.
Because the total heat removal capacity of a geothermal energy probe field depends of a number of different factors, a precise design of a geothermal energy probe field can most likely not be accomplished without computing support.
Geothermal energy probe fields can be simulated with simulation programs based on complex finite-element or finite-difference models and designed with an optimization computation. This approach, however, is time-consuming and requires from the user a deeper understanding on the level of an engineer, which typically exceeds the knowledge base of the drilling company involved in the construction of geothermal energy systems. Specialized engineering providers must then be typically engaged for the design. The associated costs can only be justified when constructing very large geothermal energy systems.
To reduce the cost and complexity associated with simulation of geothermal energy probe fields with simulation software, numerically-based software has been developed which can also be used to design a geothermal energy probe field. The significant advantage of this numerically-based software over simulation software is, on one hand, its low-cost and, on the other hand, its speed where already after several seconds of computing time usable results for designing a geothermal energy probe field can be obtained. Additionally, because the numerically-based software is based on reducing and simplifying the boundary conditions used for the design, only a small number of simple inputs is required from the user, so that application of the software requires only limited skills and can therefore also be performed by the employees of the construction companies.
Disadvantageously, however, numerically-based dimensioning software is until now available only for vertical geothermal energy probe fields. Radial geothermal energy probe fields are therefore frequently still designed by numerical calculations using the values for the specific heat removal capacity listed in the VDI Regulations VDI 4640. This numerical determination, however, is limited to heating systems sized for a maximum total heating capacity of 30 kW, is applicable only for heating applications using two geothermal energy probes and generally does not produce reliable design criteria.
Starting from the present state of the technology, it is therefore an object of the invention to provide a method with which radial geothermal energy probe fields can be easily designed with sufficient accuracy.
It would therefore be desirable and advantageous to address this problem and to obviate other prior art shortcomings by providing a method with which radial geothermal energy probe fields can be more easily designed with sufficient accuracy.